I think i could pickup a R-core transformer and compare it to my favourite potted toroid from Nuvotem/Talema.
This curiosity must be satisfied. For low power amplifiers (headphones or 2W max) they seem a valid option.
I can use ears and some basic measurements equipment - a 5,5 digits true rms DMM and a 50Mhz scope. If nothing else, this would be a good learning experience.

Edit: comparing with a screened and potted toroid would be interesting indeed.